The invention relates in general to impact printers and more particularly to the character or type-bearing print elements employed therewith.
It is well known in the art to use, is serial printers, print elements in the form of a wheel or the like. For example, see U.S. Pats. No. 2,236,663, 3,461,235, 3,498,439 and 3,651,916. A known location-layout of characters on a horizontal barrel-shaped type carrier is disclosed in U.S. Pat. No. 3,770,095.
The Diablo Corporation, a subsidiary of the present assignee, is marketing a serial printer under the tradename of Diablo Hytype I which employs a print wheel or disc-shaped type carrier. A printer of this type is disclosed in U.S. patent application filed Sept. 4, 1973, in the name of Andrew Gabor, Ser. No. 394,072, entitled "High Speed Printer With Intermittent Printer Wheel With Carriage Movement", being a continuation of an application filed Feb. 25, 1972, Ser. No. 229,314, the disclosure of which is incorporated by reference into this specification.
The Diablo Hytype I printer is enjoying commercial success as a serial printer in such applications as communication terminals, computer output devices, etc. With reference to FIG. 1 the print wheel employed in the Diablo Hytype I printer comprises a central hub section with 96 spokes or beams emanating therefrom and disposed in a circle concentric with the axis of rotation of the print wheel. A particular character slug is located at the outer end of each of the spokes. Print wheels are available for print-out at 10 or 12 pitch with all characters being allocated a width of approximately 0.0940 inches and 0.0830 inches, respectively, with the actual physical dimensions of the characters being less than the allocated values.
The character layout (FIG. 2) for the 10 and 12 pitch print wheels for the Diablo Hytype I printer was determined by use of a computer in conjunction with the "Statistical Usage of the 37 Characters in Average English" as shown in FIG. 3. Based on the character usage shown in FIG. 3, the primary objective was to layout the characters around the periphery of the print wheel so as to allow the fastest print speed possible. It can be seen that the faster printing would occur when the print wheel was required to rotate each character to be printed the shortest possible distance of rotation to a printing station.
The print wheel is rotated by a servo mechanism to position selected characters opposite a hammer and ribbon at the printing station. With reference again to FIG. 1, starting with the zero position which is the home position, which refers to home in the servo mechanism as well, each time the print wheel is moved so that a different character is positioned for printing at the printing station (which is the home position), the print wheel must be moved so that the centerline associated with the desired character is aligned to the print or home position. The diameter of the circle formed by a line connecting the centers of the characters of the print wheel for the Diable Hytype I printer is selected to be about 2.870 inches. The arc between character centerlines is about 3.degree. 45'; therefore, for these parameters the length of the arc (at the center of the characters) between centerlines is about 0.0940 inches. The length of the arc at the bottom of a character will be less than that at the center or top of a character because of the reduction in the diameter associated with the arc at that location.
The escapement of the carriage of the Diable Hytype I printer along the platen is limited to increments of 1/60th of an inch; therefore, the characters must be of the proper width to allow for correct escapement. The character width must be somewhere in the zone of 1/60th of an inch or a multiple thereof and also be positioned on the print wheel so that when you have two characters spaced together, they do not overlap. For example, for the 12 pitch print wheel, if the width of any adjacent characters is increased from the allocated 0.0830 inches to a value greater than about 0.0940 inches, then the adjacent character slugs will interfere with and contact each other. The width of the print hammer is 0.060 inches which is less than the 0.0830 inches allocated for each character space around the 96 spoke print wheel; therefore, the hammer will not overlap a character slug and contact more than one character slug during a printing operation.
The 96 spoke print wheel employed in the Diablo Hytype I printer is formed of a thermoplastic by the process of one-piece injection molding. For structural strength, the character relief depth is about 0.015 inches and the character rake angle (the rise angle with respect to vertical) is about 15.degree., which causes the character slug to be wider and occupy more area than the area occupied by the character itself; the width of the character slug is limited by the available space of about 0.0940 inches between the centerlines of adjacent spokes.
The Diablo Hytype I printer is enjoying commercial success as an electronic printer capable of high speed and versatile operation. However, in certain applications, such as automatic text editing typewriter applications in the office environment, additional features and capabilites are desired. One such feature is the ability to print in a proportional space mode in addition to the 10 and 12 pitch modes. Another feature is the ability of the typist to view the typewritten material without such view being obscured by the print wheel. The print wheel of this invention provides proportional space printing capability in the operating conditions mentioned above.
Accordingly, it is a primary object of the instant invention to develop a print wheel suitable for proportional space printing.
Another object of our invention is to provide a print wheel suitable for proportional space printing which will not substantially reduce printing speed from that for 10 and 12 pitch print wheels.
A still further object of our invention is to provide a print wheel suitable for proportional space printing in the environment of automatic text editing typewriter applications.
Other objects and advantages will be evident from the specification and claims and the accompanying drawing illustrative of the invention.